A hybrid rocket engine or motor is a cross between a solid propellant rocket motor and a liquid propellant rocket motor. In its more common configuration, a hybrid utilizes a fluid oxidizer to burn a solid fuel element. The hybrid rocket propellant can be ignited by an igniter such as an electrically generated spark, or by initial injection of an ignition fluid which exothermically reacts with the liquid oxidizer. A "reverse" hybrid, on the other hand, applies a combustible liquid fuel to a solid oxidizer. This application relates to the solid fuel/fluid oxidizer form of hybrid which, generally, is well known. Many examples of various hybrid rocket configurations appear, for example, in the patent literature. Representative are U.S. Pat. Nos. 4,424,679; 3,789,610 3,782,112; 3,323,308; 3,302,403; 3,274,771; 3,214,906; 3,164,093; 3,178,885; 3,144,751; 3,142,152; 3,137,128; 3,132,475; 3,116,599; 3,115,007; 3,083,527; 3,068,641; 2,984,973; 2,791,883; and 2,753,801.
A more recent example of a hybrid rocket motor is disclosed in commonly owned U.S. Pat. No. 5,119,627, issued Jun. 9, 1992.
Some of the more well known advantages of a hybrid rocket motor over a purely solid or purely liquid fuel rocket motor are: (1) the complete separation of fuel from the principal oxidizer, thus eliminating the potential for uncontrolled mixing; (2) the capability of optimizing the combination of propellant ingredients regardless of whether these are solid or liquid, and (3) the capability of easily stopping and restarting the motor. In addition to its on-off capability, the engine is easily throttleable since there is only one liquid component. In addition, since the solid fuel component need not contain any oxidizer, it is easily mass produced under less hazardous conditions and at less cost.
A conventional hybrid rocket motor includes a hollow housing or combustion chamber in which an elongated solid fuel component, or "grain", is secured. The solid fuel grain may have a "wagon wheel" cross sectional shape as disclosed in the above-identified U.S. Pat. No. 5,119,627, with a central hollow hub, an outer rim, and a plurality of spokes joining the hub to the rim. The wedge-shaped spaces between the spokes, known as combustion "ports", extend the axial length of the grain, thereby allowing combustion gas to flow through the grain.
The liquid or gaseous oxidizer is provided in a tank or container mounted forward of the fuel grain, and is caused to flow through the fuel grain passages. Ignition causes combustion of the fuel-oxidizer mixture at the exposed surfaces of the fuel grain, resulting in the generation of thrust as the high pressure combustion products are discharged through the rocket nozzle.
A significant problem inherent in conventional hybrid rocket motor design is that it is difficult to burn all of the solid fuel grain in a controlled manner. When it is attempted to burn the entire fuel grain, a point is reached where a central unburned portion separates from the main body from the grain, and is moved by the combustion gas flow into the nozzle area, causing a major reduction in thrust, and potentially significant damage to the nozzle and/or rocket. This may be avoided by burning the fuel grain only down to a point at which the undesirable separation will not occur. This is not a completely satisfactory solution, however, since the remaining portion of the fuel grain, and hence also precious space and weight in the rocket motor, is wasted.
While the wagon wheel grain configuration can provide an optimal ratio of exposed surface area to cross sectional area, the disadvantage of the wagon wheel design is that because of the slow burning rate of the fuel, the webs become very thin during the last portion of the burn and again, subject to separation. It has been attempted to reinforce the wagon wheel fuel grain by incorporating solid stiffening sheets in the spoke or web portions of the grain. This too has not proven satisfactory since the fuel grain tends to separate from the solid sheets during burning.